Drill pipes used in drilling wells for oil, natural gas, and the like have been connected by tool joints. In order for the tool joints to transmit high torque required during drilling, an outer diameter portion thereof is formed to be greater than an outer diameter of a pipe body, while an inner diameter portion thereof is formed to be smaller than an inner diameter of the pipe body. To this end, generally, a make-up torque value during joining a pin and a box of the tool joints is required to be several times a make-up torque value for casing or tubing used in wells for production of oil, natural gas, and the like.
On the other hand, for the number of times of make-up/break-out operations of the pin and the box of the threaded joints for the casing or tubing used in wells for production, the number of tripings is not so many. Therefore, for anti-galling (scoring) evaluation testing, International Organization for Standardization standard ISO13679 defines acceptance/rejection determination in performance evaluation for 2 times of make-up/break-out operation in the casing and for 9 times of make-up/break-out operation in the tubing. However, the drill pipes require drill bit replacement according to drilling conditions of each type such as geological strata, well inclination, depth, and the like. Further, there is no ISO standard definition for the drill pipes, but the galling resistance is expected to be not less than 25 trips, and more preferably not less than 50 trips.
For the casing or tubing, a lubricating grease (or dope) to be applied to the pin and the box of the threaded joints has been used for anti-galling, and also a surface treatment such as plating has been employed (see here bellow patent literatures 1 to 7). However, spreading due to tool joint cleaning, excess lubricating grease deposition on well bottom due to coating, rig pollution emission in workplace, etc. may have adverse effects on the environment. Therefore, for environmental consideration, alternative surface coating treatment using no conventional screw lubricating grease, so-called “grease-free” or “dope-free”, i.e., with no lubricating grease (nor dope) to be applied to the pin and the box of the threaded joints, has recently been put into practical use.
Following documents have been identified that relate to the said technical field: